The cost of photovoltaic (PV) solar cell production for terrestrial applications has declined 7-fold since 1980 through, innovation in manufacturing processes and improvements in product performance. However, further reductions in manufacturing costs are necessary before we can expect widespread use of PV solar cells in the renewable energy market. Accordingly, there is still a need for further cost savings in the manufacture of solar cells by improved process technology.
A common feature of all solar cells is the requirement of metal contacts to be applied to both the positive and negative surface of the device to carry the photo-generated current. The contacts must be robust, highly conducting, of low-cost and above all, simple and efficient to fabricate. Copper plated contacts are used in the fabrication of PV solar cells. The high electrical conductivity of copper is ideal for this application but the current deposition process by electroless plating is somewhat slow and inefficient. Furthermore the cost and handling of large amounts of chemicals is an increasing problem connected with electroless copper plating.
PV solar cells are generally based on the use of a doped semiconductor material. In one type of solar cells silicon is used as the semiconductor. This kind of solar cell typically comprises a pre-manufactured p-type doped silicon wafer. For the preparation of a solar cell this wafer is doped from the light incident surface to form n-type silicon at the surface. In this way a gradient interface between n-type and p-type silicon, termed a p-n-junction, is established. The p-n-junction produces the electrical field which makes the charge carriers move in one direction. To be able to conduct the electrical current away from the cell, metal contacts are provided on the cell. These contacts act as the positive and negative contact to the cell. However, by arranging metal contacts on the light incident surface these contacts reduce the active area on the light incident surface of the solar cell and thus reduce the efficiency of the cell. Accordingly it is important to minimize the shading effect of these metal contacts.
In U.S. Pat. Nos. 4,726,850 and 4,748,130 and the corresponding AU patent No. 570,309 (Green and Wenham) from 1984 a buried grid solar cell is disclosed in which the metal contacts on the light incident surface are embedded in grooves in the surface to reduce the shading effect and to improve the electrical contact with the semiconductor. In the patent specifications a number of methods for providing the metal contacts are listed. These include: sweeping of silver paste into the grooves, solder dipping and electroplating. However, the electroplating process is not exemplified in the patent specifications.
Use of a conventional electroplating process for the preparation of an embedded contact material in the type of grooves used in the above known buried grid solar cell would lead to an undesired formation of voids in the internal space of the grooves when the metal contact material is built up on exposed surfaces. Such voids will reduce the effectively as an electrical conductor. Furthermore the contact material will also build up on the electrically insulating light transparent layer in the areas adjacent to the grooves shadowing for the incident light to the solar cell and reducing its efficiency. Thus, although use of electroplating for the preparation of the embedded contacts was mentioned by Green and Wenham in 1984 this approach has not been further developed before the present invention.
In the electroplating industry it is common practice to use specific additives in the electroplating baths. Among such per se well known additives used in electrolytic copper plating are levelling additives. Levelling additives ensure a laminate growth during the build up of the electroplated layer and they are effective to level minor scratches in the basic material being electroplated. Such scratches are normally in the order of 0.1 to 5 μm width and 0.1 to 5 μm depth.
Another type of well known additives used in electrolytic copper plating are suppressing additives acting as diffusion controlled plating inhibitors. Such suppressing additives inhibit the build up of metal in areas having higher field intensity, such as at elevated areas being closest to the anode.
In Plating & Surface Finishing, March 2000, pp. 81-85, Mikkola et al. disclose a copper electroplating process using brighteners, levellers and suppressing agents for void-free gap fill of trenches of sub-micron dimensions used for interconnects in the area of semiconductor devices. In this article it is stressed that control of current density and additive levels has a dramatic effect on the gap fill mechanism. However, no detailed information concerning the specific composition of the electrolytic baths is given, and no information is given in case of bigger grooves such as grooves being about 20-50 μm in depth and 10-30 μm in width.
Thus, prior to the present invention a process for providing electrical contacts having good electrical conductivity and no additional shading beyond that defined by the groove dimension (width) on a buried grid solar cell, was needed.
It has now been found that it is possible to completely fill the grooves in a solar cell of the type in which grooves typically have a depth of 20-50 μm and a width of 10-30 μm using a simple electroplating technique involving a conventional electroplating bath with a special combination of per se known additives obtaining effective embedded conducts without voids and no overplating on the light incident surface outside the grooves.